Unless you are going to have them in the bottom half of your batting order or if you don't mind playing fatigued players (< 100%), I'd recommend getting another backup OF (especially if the OF with 105 PA is also your primary pinch hitter).

you have plenty of PA. if they do fall below 100% it's very fixable, just bat them lower in the lineup for a few games. or let an infield backup play OF for a game. if your backup gets fatigued he'll stop pinch hitting and the problem should fix itself.

I still think you'll be fine with the 2272 total PAs - an amount that includes the 10% bonus. It averages out to 757 per OF position (2272/3= 757). Presuming the guys don't bat 1, 2, & 3 in the order and your back up isn't used much as a PH (set accordingly, the guys should be able to play at 98-100% all the time. It depends on the park and, as mentioned, their position in the order but the latter is something you can manage as you move through the season. Just be sure the 4th guy isn't used up by pinch hitting a lot.